1. Field of the Invention
This invention relates to a cooling or refrigeration system comprising a plurality of individual compressors and more specifically to operating such a system comprising a plurality of compressors of unequal capacity in such a manner so as to selectively energize the individual compressors to efficiently match the load while avoiding stressful cycling of the compressors.
2. Background of the Invention
Cooling systems generally comprise a condensor coil, a receiving vessel for the condensed liquid from the condensor coil, an expansion valve, an evaporator coil and a compressor connected to the condensor coil. Improvements over the simplest form of cooling system just described have included multiple compressors, rather than only one, and alternating or selecting their use in accordance with demand so as to use only enough compressor capacity sufficient for demand and so as to minimize wear on the compressors and compressor contactors.
It is also known that there are advantages to operate a multiple compressor system wherein the compressors are commonly piped. Further, it is known that it is sometimes advantageous to operate a system of commonly-piped multiple compressors of unequal capacity. By selecting between the compressors using a logic circuit it is possible to selectively match demand from a range of compressor operation wherein only the smallest capacity compressor is energized to compressor operation wherein all of the available compressors are utilized. By applying binary logic to program the capacity selection device, it is possible to select all of the available incremental combinations available as represented by the individual compressors. In addition, there are other procedural programming schemes available for incrementally selecting the compressors.
It is known that it is stressful on the compressors in the system when many of them are cycling or changing state of energization at the same time. That is, surges and excessive wear of compressors and contactors are caused by cycling together three or four compressors of a 4-compressor system. Cycling only one or two compressors at a time does not cause the same degree of wear.
It is also common in a multiple compressor system where most of the compressors are of unequal capacity to have two of the same capacity, or, in the system where there are a large number of unequal capacity compressors, to have a few duplications of capacity included. It is stressful to cycle the same compressors on and off compared with another of the same capacity if every time that capacity-sized compressor is programmed for logic selection.
Therefore, it is a feature of the present invention to provide for an improved selection of the individual compressors in a commonly-piped, multiple compressor system employing largely unequal capacity compressors wherein no more than a minimum number of compressors are cycled through a change of state energization at the same time.
It is another feature of the present invention to provide for an improved selection of the individual compressors in a commonly-piped, multiple compressor system employing largely unequal capacity compressor, but where there is also at least one duplication of capacity among the compressors, so as to provide for a minimum number of simultaneous change-of-state energizations while also equalizing the cycling among the compressors of the same capacity.